1. Field of the Invention
This invention relates to communications systems, and in particular to adaptive rate communications systems that suffer time varying noise conditions, such as Annex C ADSL (Asymmetric Digital Subscriber Line) systems.
2. Related Art
In ADSL systems (e.g., using the G.992.1 and G.992.2 standard, etc.), data processing generally might be byte oriented (e.g., octet oriented). All of the bit-level processing in both the fast and interleaved transmit data paths of such systems is performed in groups of 8. The 8-bit bytes from both paths are combined and inserted into symbols in the tone ordering block (cf. FIG. 5-1 in G.992.1). Therefore, the number of bits per symbol in such systems always is a multiple of 8. One problem with such systems is that some data capacity is wasted because of the inherent 8-bit data restriction.
Annex C ADSL operates over twisted pairs that are bundled with Japanese ISDN-carrying twisted pairs. FIG. 1 is a plot depicting TCM (Time Compression Multiplex)-ISDN crosstalk noise. The crosstalk noise 101 generated by the ISDN twisted pairs is time varying, and is synchronous to a clock, called a TTR clock, having a time period 102. The TTR clock is an example of a noise clock.
Unlike an Annex C ADSL system, an Annex A ADSL system has no means of synchronization to a TTR clock and, indeed, cannot handle a time varying noise source. Thus all computations in an Annex A ADSL system such as the computations used for the derivation of equalizers (gain adjust, bit allocation, etc.) are based on a continuous average measure of the noise environment.
By contrast, an Annex C ADSL system implements two different receivers, each optimized to the characteristics of its associated noise phase. While Annex C ADSL adapts better to time varying noise than Annex A ASDL, a drawback with Annex C ASDL is that the instantaneous bit rate varies in each phase. In order to achieve a constant bit rate, significant buffering is needed, which causes latency added to the data transmission. In some applications, the additional latency is unacceptable. Thus a fallback position is to use an Annex A ASDL solution, where disadvantages in handling time varying noise are to be expected.